Abstract: Horizontal well drilling is increasingly being widely applied. However, sonic logging-one of the most important reservoir assessment methods-tends to be affected by the borehole conditions in the application of horizontal wells,leading to notably reduced application effects.In view of this fact,this paper conducts the forward simulation of the radial and axial size changes induced by borehole enlargement and the changes in enlargement positions using the three-dimensional finite difference method,with the focus on the effects of borehole enlargement on the waveform amplitude,arrival times,and velocities of waves in the sonic logging of horizontal wells.The results are as follows.In the case that the borehole enlargement occurred between the sonic transmitter and the nearest receiver,the changes in borehole enlargement parameters had no effect on the measured results of the compressional and shear wave velocities but led to the notable decrease in waveform amplitude.The amplitude of compressional and shear waves decreased with an increase in the thickness and length of the enlargement cylinder but was almost not affected by the distance between the sonic transmitter and the expansion section.The compressional wave arrival time increased with an increase in the thickness and length of the enlargement cylinder but slightly decreased with an increase in the distance between the sonic transmitter and the expansion section.This study clarifies the laws of the effects of borehole enlargement on the sonic logging response of horizontal wells,and it will provide guidance and assistance for further research on the methods for influencing factor correction in the sonic logging of horizontal wells.